This phase contains the steps to solidify and test your idea. If people don't like it here, they probably won't like it as a 200 page novel.
☐ Write a Hook
What is your book about? Is it a good idea? Is it cliché or been done a million times? This is where you figure this out. Do this first. The hook is a one sentence summary of the concept. Picture the blurb you would see in a TV listing — not much space. If you can't describe it in one sentence, it's not thought out enough.
A good discussion on how to write a great hook is in the book Save the Cat by Blake Snyder.
☐ Test the Hook
After this, say it to everyone you know and watch their reactions. Do they think it's a good idea? Would they read it? If so, you have an idea worth developing.
☐ Write Synopsis
Now you can add more meat on this by writing the synopsis. This is the one to three paragraph description that would appear on the back of the book. It fleshes out the theme and should seek to "sell" the book. First impression is here.